Vehicle capacitive detection systems comprising capacitive sensors operated in loading mode are nowadays widely used, for instance for the purpose of detection of vehicle seat occupancy. The capacitive sensors may be designed as sense-only capacitive sensors having a single sense electrode. Alternatively, they may be designed as guard-sense capacitive sensors, having a sense electrode and a guard electrode proximally arranged and mutually insulated from each other.
For example, document JP-H11-78655 describes a vehicle seat occupancy detecting apparatus including electric field sensors. A high frequency oscillator whose frequency is about 100 kHz is connected via a resistor to an antenna electrode arranged on an automobile seat. By this, a differential AC electric field is generated between the antenna electrode and the automobile ground, so that a load current corresponding to the AC electric field flows through the resistor. The AC load current is converted by the resistor into an AC voltage which is then transmitted by a voltage buffer to a detector including a bandpass filtering function which generates a DC output voltage.
If an occupant is seated on the seat, the current flowing between the antenna electrode and the automobile ground is increased, indicating the presence of the occupant on the seat.
An occupant detecting apparatus for detecting an occupant seated on the passenger seat of a vehicle is described in US application 2002/0038947. The occupant detecting apparatus comprises a plurality of electric field sensors provided in a bottom part and a rear part of the seat, and also a mechanical load sensor and an acceleration sensor. The electric field sensors are connected to a control unit comprising a high frequency oscillator, a resistor, a voltage buffer and a detector. Antenna electrodes are selected by the selectors and connected between the resistor and a voltage buffer. An analog-to-digital converter performs an analog-to-digital conversion on output signals of the selected electric field sensors. Digital output signals of the electric field sensors, the node sensor and the acceleration sensor are connected to a central processing unit, which determines a seat occupancy based on the sensor output signals, following predetermined criteria.